Synthesis and applications of carbon allotropes in heterogeneous catalysis and in optical sensors for the detection of heavy-metals in water

In addition to the macroscopic forms generally known, such as graphite or diamond, carbon is also capable of forming very different nanostructures which have undergone enormous development in many technological applications, over the last ten years The resulting structural diversity of molecules and organic compounds is accompanied by a broad variety of chemical and physical properties that can be also modulated and tailor designed by the tools of modern synthetic chemistry with the purpose of generating new target systems. Currently synthetic carbon allotropes represent a growing class of interesting architectures with outstanding material properties. In particular, quantization effects have been extensively developed starting from bulk materials, in order to modify their properties and make them dimension-dependent. This approach has aroused particular interest especially in the field of innovative carbon-based nanomaterials, such as Graphene Quantum Dots (GQDs) and Nanodiamonds (ND) which attracted growing interest, in the fields of medicine, biology, catalysis, energy and sensors [1,2] . As regards GQDs, they represent a class of luminescent nanomaterials that compared to molecular fluorophores subjected to photobleaching, have greater stability and better biocompatibility. A large literature is dedicated in particular to Graphene Oxide Quantum Dots (GOQDs) since the intrinsic presence of functional groups containing oxygen gives them a hydrophilic character that stabilizes nanoparticle suspensions in an aqueous medium without further functionality [3] . However, until now, few studies have been carried out on the sensorial properties of GOQDs, in order to evaluate their potential practical applications. The developed synthetic techniques developed are mainly aimed at the dimensional control and an effective and simple realization of GOQDs, but to date variable and contradictory results are found in the literature which could probably depend on the dimensional differences, the content of defects and functional groups, depending on the different preparation methods and the starting compounds used for the synthesis of GOQDs [4] . In recent years, increasing attention has been paid to another form of non-molecular carbon such as detonation nanodiamonds (DNDs) thanks to the attractive properties of NDs closely related to confinement effects and singular surface energy values [5] . NDs, in addition to having properties similar to those of diamond, such as particularly high chemical stability, rigidity, hardness and resistance, also show the typical advantages of nanomaterials, such as a large surface, small size and high adsorption capacity. In particular, the NDs produced by detonation (DNDs) can be easily connected by interactions or covalent bonds to different molecules and electrostatic and / or hydrophobic bio-compounds [6] . Precisely for this reason, DND can act as a non-traditional support for the construction of metal-containing catalysts and for the study of their catalytic activity in various chemical reactions. Despite the NDs have a remarkable importance in optical, biomedical, lubrificants, composites, lubricants and other applications [7,8] , there is still little literature regarding their use in heterogeneous catalysis.